Artificial intervertebral disc

ABSTRACT

An intervertebral prosthesis includes a disc member dimensioned for insertion within an intervertebral space between adjacent vertebrae to replace at least a portion of an intervertebral disc removed therefrom. The disc member has sufficient rigidity to support the adjacent vertebrae in spaced relation, and defines a longitudinal axis extending the height of the disc member and a lateral axis transverse to the longitudinal axis. The disc member includes an exterior wall which has a slit defined therein. The slit defines a longitudinal component of direction and a lateral component of direction. Preferably, the exterior wall includes a plurality of helical slits, adjacent slits being disposed in at least partial overlapping relation to define an overlapping region. Upon insertion of the disc member within the intervertebral space with the support surfaces in contacting engagement with respective vertebral portions of the adjacent vertebrae, forces exerted by the vertebral portions on the support surfaces are transferred along the exterior wall through the overlapping region.

BACKGROUND

1. Field of the Disclosure

The present disclosure generally relates to apparatus and techniques fortreatment of spinal disorders, and, in particular, relates to anartificial intervertebral prosthesis which restores both the height andshape of the intervertebral disc space following the removal of adamaged or diseased intervertebral disc while maintaining the naturalbiomechanics of the spinal motion segment.

2. Description of the Prior Art

The objective in inverterbral disc replacement is to provide aprosthetic disc that combines both stability to support the high loadsof the patient's vertebrae and flexibility to provide the patient withsufficient mobility and proper spinal column load distribution. Inattempting to strike this balance, generally, four basic types ofartificial intervertebral discs for replacing a part or all of a removeddisc have been developed, namely, elastomer discs, ball and socketdiscs, mechanical spring discs and hybrid discs. Elastomer discstypically include an elastomer cushion which is sandwiched between lowerand upper rigid endplates. The elastomer discs are advantageous in thatthe elastomer cushion functions similar in mechanical behavior to theremoved intervertebral disc tissue. However, a disadvantage of this disctype is that the elastomer cushion experiences long term in-vivoproblems stemming from microcracking, which detracts from its usefulnessas a replacement option. Furthermore, attachment of the flexibleelastomer cushion to rigid endplates presents additional difficulties.Examples of elastomer discs are disclosed in U.S. Pat. No. 5,702,450 toBisserie; U.S. Pat. No. 5,035,716 to Downey; U.S. Pat. No. 4,874,389 toDowney; and 4,863,477 to Monson.

Ball and socket discs typically incorporate two plate members havingcooperating inner ball and socket portions which permit articulatingmotion of the members during movement of the spine. The ball and socketarrangement is adept in restoring “motion” of the spine, but, is poor inreplicating the natural stiffness of the intervertebral disc.Dislocation and wear are other concerns with this disc type. Examples ofball and socket discs are disclosed in U.S. Pat. No.: 5,507,816 toBullivant and U.S. Pat. No. 5,258,031 to Salib et al.

Mechanical spring discs usually incorporate one or more coiled springsdisposed between metal endplates. The coiled springs preferably define acumulative spring constant sufficient to maintain the spaced arrangementof the adjacent vertebrae and to allow normal movement of the vertebraeduring flexion and extension of the spring in any direction.Disadvantages of the mechanical spring disc types involve attachment ofthe coiled springs to the metal end plates and associated wear at theattachment points. Examples of mechanical spring discs are disclosed inU.S. Pat. No. 5,458,642 to Beer et al. and 4,309,777 to Patil.

The fourth type of artificial intervertebral disc, namely, the hybridtype incorporates two or more principles of any of the aforedescribeddisc types. For example, one known hybrid disc arrangement includes aball and socket set surrounded by an elastomer ring. This hybrid discprovides several advantages with respect to load carrying ability, but,is generally complex requiring a number of individual components.Furthermore, long term in vivo difficulties with the elastomer cushionremain a concern as well as wear of the ball and socket arrangement.

Another type of intervertebral disc prosthesis is disclosed in U.S. Pat.No. 5,320,644 to Baumgartner. With reference to FIGS. 1-3, theBaumgartner '644 device is a unitary intervertebral disc member 1 madefrom a strong, elastically deformable material. The disc member 1 hasparallel slits 5 each arranged at a right angle to the axis of the discmember. The parallel slits 5 partially overlap one another to defineoverlapping regions 6 between adjacent slits. The overlapping regions 6create leaf springs 7 for the transmission of forces from one vertebralattachment surface to the other. In regions of adjacent slits 5 wherethey do not overlap the spring action on the leaf springs 7 isinterrupted by fixation zones 9 of solid prosthesis material. The forcesacting on the intervertebral disc are transmitted from one leaf springplane to the next leaf spring plane via the fixation zones 9.

However, the load paths are inherently abrupt with highly localizedtransfer of load through the sparsely placed fixation zones 9. There areeven instances where the entire load is carried through a singlefixation zone 9 in the center of the disc. The abrupt load paths canlead to high stress regions, which can detract from the appropriatebiomechanical performance, i.e., strength, flexibility, andrange-of-motion, of the prosthesis.

The need therefore exists for a prosthetic disc which is easy tomanufacture and provides the proper balance of flexibility and stabilitythrough improved load distribution.

SUMMARY

Accordingly, the present disclosure is directed to an intervertebraldisc prosthesis for insertion within the intervertebral space betweenadjacent vertebrae to replace at least a portion of an intervertebraldisc removed therefrom. The intervertebral prosthesis includes a discmember having a longitudinal axis extending the height of the discmember and a radial axis transverse to the longitudinal axis. The discmember includes an external wall having at least one slit therein. Theat least one slit has a first component of direction and a seconddifferent component of direction and facilitates transfer of load alongthe exterior wall.

Preferably, the exterior wall includes a plurality of helical slits,adjacent slits being disposed in radial relation with respect to thelongitudinal axis whereby load transfer occurs along the exterior wall.The slits give the exterior wall flexibility consistent with the naturalintervertebral disc.

The disc member may further include an inner cavity. Preferably, theslit(s) extends from an outer wall surface of the exterior wall to aninner wall surface thereof in communication with the inner cavity. Firstand second longitudinally opposed support surfaces are disposed at thelongitudinal ends of the disc. The support surfaces are dimensioned tosupportingly engage vertebral portions of respective vertebrae. At leastone of the first and second support surfaces has an opening extendingtherethrough in communication with the inner cavity.

An end cap may be releasably mounted to the support surfaces and atleast partially positionable within the opening in the support surface.The end cap may include an inner opening dimensioned to reduce rigiditythereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment(s) of the present disclosure are described hereinwith reference to the drawings wherein:

FIGS. 1-3 illustrate a prior art intervertebral disc prosthesis;

FIG. 4 is a perspective view of the artificial intervertebral prosthesisin accordance with the principles of the present disclosure, includingthe disc member and the end cap(s) mounted to the disc member;

FIG. 5 is a perspective view of the intervertebral prosthesis of FIG. 4with the end caps removed from the disc member;

FIG. 6 is a cross-sectional view of the intervertebral prosthesis ofFIG. 4;

FIG. 7 is a view illustrating a portion of the vertebral column;

FIG. 8 is a view taken along the lines 8-8 of FIG. 7 illustrating theintervertebral prosthesis of FIG. 4 positioned within the intervertebralspace defined between adjacent vertebrae;

FIG. 9 is a perspective view of an alternate embodiment of theintervertebral disc prosthesis;

FIG. 10 is a perspective view of another alternate embodiment of theintervertebral disc prosthesis;

FIG. 11A is a cross-sectional view taken through the vertebral body toillustrate a top view of the fusion cage of the present disclosure; and

FIG. 11B is a perspective view of the fusion cage of FIG. 11A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, in which like reference numerals identifysimilar or identical elements throughout the several views, andreferring in particular to FIGS. 4-6, the artificial intervertebralprosthesis of the present disclosure is illustrated. Intervertebralprosthesis 100 is intended to replace part or all of the supportingfunction of a diseased intervertebral disc which had been previouslyremoved through a discectomy procedure or the like. Intervertebralprosthesis 100 is advantageously dimensioned to be positioned betweenadjacent vertebrae in supporting contacting relation with the vertebralend plates thereof to maintain the adjacent vertebrae in appropriatespaced relation while restoring the natural biomechanics (e.g.,including stiffness, range of motion, and strength) of the spinal orvertebral segment.

Intervertebral prosthesis 100 includes two basic components, namely,disc or body member 102 and first and second end caps 104, 106 which arereleasably mounted to the body member 102. Body member 102 is in thegeneral shape of an intervertebral disc (e.g., kidney-shaped) as shownand defines longitudinal axis “a” extending along the height of themember 102 and radial median axis “b” generally transverse to thelongitudinal axis “a”. An angular reference is defined by “c” as shown.(FIG. 5) Body member 102 includes first and second longitudinallyopposed (e.g., upper and lower) support surfaces 108, 110 whichsupportingly engage the respective end faces of the adjacent vertebraeupon insertion of the prosthesis, and exterior wall 112 extendingbetween the support surfaces 108, 110. Support surfaces 108, 110 areeach arcuate in configuration defining a slight outer curvature whichpreferably corresponds to the slight inward curvature of the vertebralend plates so as to facilitate positioning and retention of theprosthesis within the intervertebral space.

Body member 102 further includes a centrally located cannulation 116 ingeneral alignment with the longitudinal axis “a” and extending throughsupport members 108, 110. (FIG. 5) Cannulation or bore 116 defines aninner cavity 114 and central openings 118 of the support surfaces 108,110. In the embodiment illustrated in FIG. 4, openings 118 arecorrespondingly dimensioned to at least partially receive theirrespective end caps 104, 106. An enlarged circumferential recess 120 isdefined within each support surface 108, 110 about the periphery of eachopening 118 to receive the head portion 130 on the end caps 104, 106. Asshown, the end caps 104, 106 once inserted, are generally flush with theupper and lower surfaces 114. The end caps 104, 106 provide additionalsurfaces 134 for bone attachment and prevent bone growth into the bodymember 102. The engagement surfaces 142, 144 of the end caps 104, 106,during high load contact each other and serve several purposes: (1)prevent the exterior walls 112 from being overstressed by providing analternate load path (through the center of the disc); (2) increase theoverall stiffness of disc 100 in a similar manner as the natural discwhich becomes more rigid with high loads; and (3) prevent completeclosure of the generally helical slits 122, reducing a “pinching” effecton surrounding soft tissue. Internal bore 138 with its associate slottedopenings 140 effectively reduce the rigidity of the end caps 104, 106,so that the overall stiffness of the disc 100 will be more consistentwith the natural intervertebral disc.

With continued reference to FIGS. 4-6, exterior wall 112 has a pluralityof slits 122 defined therein which, in the preferred embodiment, extendcompletely through the exterior wall from its outer surface 124 to itsinner surface 126 in communication with the inner cavity 114. (FIG. 6)Each slit 122 is generally helical in configuration, i.e., each slit 122has a longitudinal component of direction and an angular component ofdirection as shown. These different directional components e.g. alongitudinal and lateral direction, result in a multi-directional pathfor each of the slits 122. Slits 122 are preferably disposed about theexterior wall at predetermined spaced radial locations whereby adjacentlongitudinal slits 122 are in partial overlapping arrangement. In theillustrated embodiment, five slits 122 are provided which are radiallyspaced at 72° intervals, although alternate numbers of slits and otherspaced intervals are contemplated.

The slits 122 as shown extend about 180° around the exterior wall 112,although they can extend less than or greater than 180°. Slits 122extend along the longitudinal axis “a” from a position proximal outersupport surface 108 to a position proximal outer support surface 110,and preferably extend across the median or transverse axis “b”equidistally disposed between the support surfaces 108, 110. A singlegenerally helical slit may be used, however, the preferred embodimentprovides a plurality of generally helical slits 122. The helical slits122 are displaced in a radial relation with respect to the radial axis“b” and angle “c”. The remaining load path 128 of the disc wall 112 hasa spring-like characteristic, similar to a compressive or coiled spring.The plurality of load paths 128 create a flexible disc wall 112 andallow the transfer of loads between upper support surface 108 and lowersupport surface 110, in a continuous manner without abrupt load paths.

Although helical slits are shown, it is also contemplated that othermulti-directional slits, i.e. having a lateral and longitudinalcomponent of direction can be utilized. This can include slits that aresmooth, piecewise smooth, open-looped, etc.

With further reference to FIGS. 4-6, end caps 104, 106 each definecircumferential ledge or head portion 130 and main portion 132 ofreduced dimension. End caps 104, 106 are at least partially receivedwithin central openings 118 of support surfaces 108, 110 in a mannerwhereby circumferential head portion 130 resides in correspondinglydimensioned circumferential recess 120 of the support surface 108, 110and main portion 132 extends within the cannulation 116. The outersurface 134 of each end cap 104, 106 is preferably arcuate in shapegenerally corresponding to the arcuate configuration of the outersupport surface 108, 110 to form a smooth transition from the outersupport surfaces 108, 110 to the end cap. End caps 104, 106 each furtherinclude an indentation 136 defined in outer support surface 134 forattaching an instrument to releasably hold the end cap 104, 106 duringinsertion into the body member's 102 central openings 118. Indentation136 is generally clover-shaped although other shapes are contemplatedincluding rectangular, hexagonal, etc. to receive appropriateinstrumentation. Main portion 132 of each end cap 104, 106 has a centralinternal bore or cavity 138 which extends through its outer wall todefine a plurality (e.g., 4) of radially arranged slotted openings 140.Internal bore 138 with its associated radial openings 140 effectivelyreduce the rigidity of the respective end caps 104,106. The caps canalternatively have helical slits instead of openings 140 to furtherreduce stiffness.

The components of intervertebral prosthesis 100 are fabricated from asuitable rigid material including stainless steel, titanium or asuitable polymeric material. Preferably, the body member 102 ismonolithically formed as a single unit although it is envisioned that inan alternate embodiment the body member 102 is composed of separatecomponents, each of which would have the structural features, e.g.helical slit and inner cavity, discussed above. For example, threecomponents can be utilized which when placed in juxtaposition in theinvertebral space form the kidney shape of FIG. 4.

Insertion of the Artificial Intervertebral Disc

With reference to FIGS. 7-8, the insertion of the artificialintervertebral disc will be discussed. The intervertebral space “i”defined between adjacent vertebrae “Vd₁, V₂” is accessed utilizingappropriate retractor instrumentation or the like. Thereafter, a partialor full discectomy is performed to remove the diseased portion of thedisc. The adjacent vertebrae “V₁, V₂” are distracted with appropriatedistractor instrumentation to expose the intervertebral space. Theartificial intervertebral prosthesis 100 is then positioned within theintervertebral space “i”. Upon placement, the upper and lower supportsurfaces 108, 110 engage the respective vertebral end plates of theadjacent vertebrae in supporting relation therewith. As noted above, thearcuate contours defined by the outer surfaces 134 of the end caps104,106 and outer surfaces of the upper and lower support surfaces108,110 approximates the arcuate contour of the vertebral end plates tosnugly fit within the adjacent vertebrae and facilitate retention withinthe intervertebral space.

As indicated hereinabove, the artificial intervertebral prosthesis 100is characterized by having sufficient rigidity to maintain the adjacentvertebrae in spaced relation while possessing adequate flexibility topermit flexural movement of the vertebral column. The loads applied tothe intervertebral prosthesis 100 are transmitted between the upper andlower support surfaces 108, 110 through the exterior wall 112 alonggenerally continuous paths via the helical slit 122 arrangement and theresulting plurality of load paths 128.

Alternate Embodiment(s)

FIG. 9 illustrates an alternate embodiment of the present disclosure.Intervertebral prosthesis 200 includes disc or body member 202 which issubstantially similar to body member 102 of the embodiment of FIG. 4.However, in accordance with this embodiment, end caps 104, 106 areeliminated such that the support surfaces 208, 210 are continuous. Also,there are no openings 118 within the support surfaces as in theembodiment of FIG. 4 (see surfaces 108, 110). The cavity or bore (notshown) extends internally between surfaces 208, 210. Thus, in accordancewith this embodiment, the prosthesis is a single monolithically formedunit. Prosthesis 200 can include internal “caps” which contact eachunder heavy load to thereby function in a similar manner to the caps104, 106 of prosthesis 100 of FIG. 4.

FIG. 10 illustrates another alternate embodiment of the presentdisclosure. Prosthesis 300 is substantially similar to prosthesis 100 ofFIG. 4, however, in accordance with this embodiment, exterior wall 312includes a single continuous helical slit 302 which extends from aposition adjacent upper support surface 308 to a position adjacent lowersupport surface 310. The load paths are designated by reference numeral328. This provides more flexibility. Continuous slit 302 definesoverlapping regions wherein longitudinally displaced portions of thecontinuous slit are in partial overlapping relation. These overlappingregions of the continuous slit 302 also provide for a continuous loadtransfer from upper support surface 108 to lower support surface 110,the benefits of such arrangements being discussed hereinabove. End caps104 and 106 can optionally be provided.

Fusion Cage with Helical Slit(s)

The present disclosure also includes a unique fusion cage illustrated inFIGS. 11A and 11B and designated generally by reference numeral 500. Inthe use of spinal fusion cages, load sharing with the bone graft packedwithin the cage is necessary to transform the bone graft into a solidbony arthrodesis. With current fusion cages, such as those made oftitanium alloy, the cage is rigid, resulting in the cage as the dominantload path during the fusion process.

The fusion cage 500 of the present disclosure is preferably composed ofa titanium alloy. However, the cage includes a slit configuration toreduce stiffness. That is, the helical slits 522 provide the cage withadditional flexibility so they flex under load, resulting in greaterload sharing with the graft. As can be appreciated, fusion cage 500 hasthe identical helical slit configuration as the prosthetic disc of FIG.4, and therefore the slit configuration will not be described again.Note that the slit design of FIG. 10 can also be utilized.

Cage 500 includes an internal cavity 502 to receive bone graft material“g” (see FIG. 11A). End caps (not shown) can be provided to help retainthe bone graft material and to limit flexure as described above, as longas the caps have openings communicating with the internal cavity 502 toensure contact between the bone graft material and vertebrae. Once thecage 500 is placed in the vertebral space “i” with support surfaces 508,510 contacting the vertebrae, this bone graft material inside cavity 502fuses with the adjacent vertebrae over time. As shown in FIG. 11A, aswith current fusion cages, cage 500 is smaller than the overall discspace. Although one is shown, it is contemplated that two or more cages500 can be placed side by side in the disc space.

Also, since fusion cage 500 does not fill the entire disc space, shapesother than the kidney shape of FIG. 11A and 11B are also contemplated,provided they contain the slit configuration to reduce overallflexibility

It will be understood that various modifications may be made to theembodiment disclosed herein. Therefore, the above description should notbe construed as limiting but merely as an exemplification of a preferredembodiment. Those skilled in the art will envision other modificationswithin the scope and spirit of the claims appended hereto.

What is claimed is:
 1. An intervertebral prosthesis which comprises adisc member dimensioned for insertion within an intervertebral spacebetween adjacent vertebrae to replace at least a portion of anintervertebral disc removed therefrom, the disc member defining alongitudinal axis, the disc member including a substantially enclosedexterior wall having opposed longitudinal ends for positioning adjacentrespective upper and lower vertebrae, the exterior wall beingsubstantially solid and having a plurality of slits therein to permitthe exterior wall to elastically deform when subjected to a load, atleast portions of three helical slits being disposed in overlappingrelation.
 2. An intervertebral prosthesis, which comprises a disc memberdimensioned for insertion within an intervertebral space betweenadjacent vertebrae to replace at least a portion of an intervertebraldisc removed therefrom, the disc member defining a longitudinal axis,the disc member including a substantially enclosed exterior wall havingopposed longitudinal ends for positioning adjacent respective upper andlower vertebrae, the exterior wall being substantially solid and havinga plurality of helical slits therein to permit the exterior wall toelastically deform when subjected to a load, the helical slits extendingonly about a portion of a periphery defined by the exterior wall,adjacent helical slits being disposed at predetermined spaced radiallocations with respect to the longitudinal axis.
 3. An intervertebralprosthesis, comprising a body member dimensioned for insertion within anintervertebral space between adjacent vertebrae to replace at least aportion of an intervertebral disc removed therefrom, and defining alongitudinal axis and a median transverse axis, the body memberincluding an exterior wall, an inner cavity defined within the exteriorwall and support surfaces disposed at respective longitudinal ends ofthe exterior wall, the exterior wall having a plurality of independentand non-communicating helical slits defined therein and extending tocommunicate with the inner cavity, at least one of the helical slitsextending across the median transverse axis.
 4. The intervertebralprosthesis according to claim 3 wherein the helical slits are positionedsuch that at least two adjacent helical slits are in partial overlappingrelation with respect to the longitudinal axis to define an overlappingregion.
 5. The intervertebral prosthesis according to claim 4 wherein atleast one support surface has an opening in communication with the innercavity.
 6. The intervertebral prosthesis according to claim 5 whereinthe support surfaces each have an opening therein in communication withthe interior cavity.
 7. The intervertebral prosthesis according to claim6 including first and second end caps releasably mounted to respectivefirst and second support surfaces and receivable within the openingsdefined therein.
 8. An intervertebral prosthesis, comprising a bodymember dimensioned for insertion within an intervertebral space betweenadjacent vertebrae to replace at least a portion of an intervertebraldisc removed therefrom, the body member including an exterior wall, aninner cavity and support surfaces disposed at respective ends of theexterior wall, the exterior wall having a plurality of helical slitsdefined therein and extending to communicate with the inner cavity, thehelical slits being arranged such that at least three slits are disposedin partial overlapping relation to define an overlapping region.
 9. Anintervertebral prosthesis, comprising a disc member dimensioned forinsertion within an intervertebral space between adjacent vertebrae toreplace at least a portion of an intervertebral disc removed therefrom,the disc member defining a longitudinal axis and a transverse axistransverse to the longitudinal axis, the disc member including anexterior wall portion having opposed longitudinal ends for positioningadjacent respective upper and lower vertebrae and defining a pluralityof slits therein, each slit defining a multi-directional path havinglongitudinal and transverse components of direction to provide a generalarcuate appearance thereto, the slits dimensioned to extend sufficientlywithin the exterior wall portion and arranged to define a plurality ofcontinuous load paths extending between the longitudinal ends wherebyupon implantation of the disc member within the intervertebral spaceforces exerted on the disc member are transferred along the load pathsof the exterior wall portion.
 10. An intervertebral prosthesis, whichcomprises a disc member dimensioned for insertion within anintervertebral space between adjacent vertebrae to replace at least aportion of an intervertebral disc removed therefrom, the disc memberhaving sufficient rigidity to support the adjacent vertebrae in spacedrelation, and defining a longitudinal axis, the disc member including anexterior wall and an inner cavity defined by and within the exteriorwall, the exterior wall including at least three non-communicating slitsdefined therein, at least one of the slits extending from an outer wallsurface of the exterior wall to an inner wall surface of the exteriorwall in communication with the inner cavity, the one slit being helicaldefining a general helical arrangement with respect to the longitudinalaxis.
 11. The intervertebral prothesis according to claim 10 whereineach of the slits is helical.
 12. An intervertebral prosthesis, whichcomprises a disc member for positioning within an intervertebral spacebetween adjacent vertebrae, the disc member including a substantiallysolid exterior wall having opposed longitudinal ends with respect to acentral longitudinal axis of the disc member for positioning adjacentrespective upper and lower vertebrae, the exterior wall defining aninner cavity, the exterior wall having a plurality of generalhelical-shaped slits defined therein, adjacent slits being arranged atpredetermined spaced radial locations with respect to the centrallongitudinal axis and in at least partial overlapping relation withrespect to the central longitudinal axis to define a plurality ofradially-displaced load paths for transferring a load between thelongitudinal ends of the disc member.
 13. An intervertebral prosthesis,which comprises a disc member dimensioned for insertion within anintervertebral space between adjacent vertebrae to replace at least aportion of an intervertebral disc removed therefrom, the disc memberdefining a longitudinal axis, the disc member including an exterior walldefining an internal cavity and having support surfaces disposed atlongitudinal ends of the exterior wall, the exterior wall including aplurality of independent helical slits arranged to permit the exteriorwall to elastically deform when subjected to a load, at least onesupport surface having an opening in communication with the internalcavity and an end member mountable to the one support surface tosubstantially enclose the opening.
 14. The intervertebral prosthesisaccording to claim 13, wherein the end member defines an outer wallportion, the outer wall portion having at least one slit dimensioned toreduce rigidity of the end member.